We plan to study in detail a series of "white pock" host range mutants of rabbit poxvirus (RPmu mutants) which in nonpermissive cells are blocked at many stages of viral development. As a major effort, we plan to continue our mapping studies on these mutants, which we have shown to be the result of deletions, by the use of restriction enzyme analysis. Our studies on the five mutants that we have examined have already implicated 17% of the viral genome in the host range phenonema. Our data show that these five mutants result in three different phenotypes in nonpermissive cells, offering the possiblity of correlating defined regions of the genome to a specific function. As a step in this direction, we plan to map the remaining 15 host range RPmu mutants in our collection and characterize them as to what steps in the synthesis and processing of macromolecules are blocked in the nonpermissive host and why. We also plan to extend our genetic mapping studies to include a series of "white pock" nonhost range mutants, also probably deletions, which because they readily recombine with the host range mutants, should lie elsewhere on the genome. We have already biochemically characterized two mutants which appear to be blocked at the level of translation. We propose to study the mechanism of this translation block in detail. Three additional mutants are blocked at the level of morphogenesis, and synthesized aberrant virus-like particles different from one another. We plan to characterize the aberrant particles by biophysical and electron microsopic means, as well as by assay of virion-associated enzymatic activities. Since the existence of host range mutants reflects biochemical differances between cells, we hope to use our information to begin to explore these differences which allow the selective invasion of target organs and tissues in vivo and lead to the disease state asociated with a given viral infection.